ive heard from more than one guy the closed chamber heads do not work on the smogger engines with the piston way down in the hole. you dont get the benefits from the closed chamber unless you are close to zero deck, all they got was real bad detonation. that doesnt really make a bunch of sense since the low po 383s that came with the closed chamber heads had the pistons pretty far down too. i also knew one guy that put larger valves and ported a set of 516 heads switching from similarly prepped 906 heads and he actually slowed down a little.
this is just things ive heard. i have a pile of BB heads here and i do have a set of 516s but ive always been leary of dumping a bunch of cash into them.